Human breast cancer cells, MCF-7, were grown in vitro for 2 days in the presence or absence of various doses of interleukin-2 or interleukin-1. Both of these lymphokines have direct effects on the growth of MCF-7 cells. Interleukin-1 has a strictly inhibitory effect on growth, while interleukin-2 has a biphasic effect. At high doses IL-2 inhibits MCF-7 cell growth while at low doses IL-2 actually enhances tumor cell growth.